The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, it is desired that HDDs be able to store more information in their limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components.
One attempt at miniaturizing components has led to the use of thermally-assisted magnetic heads, and a method of recording using these heads has been proposed for implementing high-density magnetic recording of at least 1 Tb/in2 in H. Saga, H. Nemoto, H. Sukeda, and M. Takahashi, Jpn. J. Appl. Phys. 38, Part 1, pp. 1839 (1999). When the recording density exceeds 1 Tb/in2 in a conventional magnetic recording device, the erasure of the recorded information by thermal vibration becomes a problem. In order to prevent this, the coercive force of the magnetic recording medium is increased. However, because the increase is limited to the magnitude of the magnetic field that can be generated by the write head, it is impossible to form recording bits in the medium when the coercive force is too high. As a solution in thermally assisted recording devices, the medium is heated by light at the instant of recording to lower the coercive force. Thus, recording to a high coercive force medium is possible, and a recording density above 1 Tb/in2 may be achieved.
As shown in FIG. 1, a portion of a thermally-assisted magnetic head 100 is shown according to the prior art to help describe the thermally-assisted recording method described above. In this thermally-assisted magnetic head, an area near to the main magnetic pole 102 for applying a magnetic field must be heated. Therefore, a waveguide 104, for example, is formed along the side of the main magnetic pole 102, and semiconductor laser light, which comes from a semiconductor laser light source 106, is guided to the area near to the front end of the main magnetic pole 102.
Various methods have been proposed for mounting the semiconductor laser light source 106. However, a method that mounts the laser light source 106 directly above the slider 108, introduces light into the waveguide 104 formed in the slider 108, and guides the light to a near-field light-generating element 110, such as a transducer, formed in the vicinity of the ABS seems to have the most promise because of an easier implementation with fewer components, a simpler configuration, and lower costs.
The operating principle of thermally-assisted magnetic recording according to this method is that during recording, the laser light source 106 emits light, and the laser light is introduced into the waveguide 104. The light introduced into the waveguide 104 is converted into near-field light that heats only a minute region in the vicinity of the surface of the magnetic recording medium 112 by the near-field light-generating element 110 to heat a local region on the medium 112. By applying the recording magnetic field for modulating the polarity in response to the recorded information simultaneously to increasing the temperature of the local region of this medium 112 via this heating to near the Curie temperature of the magnetic recording film 114 of the medium 112, the direction of magnetization of the local region is aligned in the direction of the recording magnetic field, that is, information may be recorded. To ensure long-term stability of the recorded information in this thermally-assisted magnetic recording method, the anisotropic magnetic field of the medium 112 at room temperature must be sufficiently large. Even if the recording magnetic field is applied to a region that is not heated, the magnetization of that region does not reverse, and only the magnetization of the locally-heated region is reversed. Consequently, ultra-high density recording becomes possible by limiting the size of this heated region to an extremely small region. A near-field light-generating element 110 is used as the heat source for heating extremely small regions.
The size of the near field light irradiated from the near-field light-generating element 110 is uniquely determined primarily by the shape and size of the near-field light-generating element 110 and the distance between the medium 112 and the head 100. In practice, the size of the region being recorded is changed by the temperature distribution determined by the balance between heating by the near field light and heat dispersion in the medium 112. Specifically, to record in only the intended region, the intensity of the heating by the near-field light-generating element 110, that is, the intensity of the laser light irradiated by the near-field light-generating element 110, must be precisely controlled.
For example, a method described in Unexamined Japanese Patent Application No. 2011-14214 proposes a method for precisely controlling this intensity. In this conventional example, by monitoring the increase in the temperature of the near-field light-generating element 110 by the irradiation of light or the temperature of a temperature detection element provided near the near-field light-generating element 110, the energy of the light introduced to the near-field light-generating element 110 is monitored, and based on this information, the output of the laser light source 106 is varied and driven. Specifically, the effects of various fluctuations (temperature fluctuations, fluctuations over time) can be corrected by automatic power control via feedback control to the light source 106.
In practice, however, when the oscillation wavelength of the laser is changed by controlling the temperature variations or the drive current of the laser light source 106, interference conditions in the waveguide 104 in the slider 108 change because the optical interference conditions also change. As a result, the light intensity guided to the ABS fluctuates. The oscillation wavelength of the laser light source 106 is limited to the discrete wavelengths determined by the resonator mode (longitudinal mode) of the laser oscillations, is not necessarily uniquely changed with respect to the temperature or the drive current, and exhibits behavior similar to a type of hysteresis. In addition, the phenomenon referred to as mode hopping is produced in which the oscillation wavelength is changed discretely and in an extremely short time from some longitudinal mode to another longitudinal mode when the wavelength changes, as shown in FIG. 2, according to the prior art.
The speed of this mode hopping is determined by the relaxation oscillation frequency of the laser, but this relaxation oscillation frequency is determined by the oscillation gain of the laser light source 106 and the lifetime of the photons in the laser resonator, and is an extremely fast speed, usually several gigahertz (GHz) (approximately 0.1 ns). Specifically, because the light fluctuations caused by mode hopping occur at a high speed of 1 ns or less, electrical feedback control by drive current control is essentially impossible and incredibly impracticable. In addition, although this mode hopping depends on the generation frequency caused by the temperature of the laser light source 106, the injected current, or the reflected return light, prediction is impossible because the probability of generation is essentially random. When wavelength fluctuations caused by this mode hopping occur in a conventional heat-assisted magnetic head 100, the intensity of light reaching the ABS plane, namely, the assisted light intensity, fluctuates. As a result, the recording conditions fluctuate at high speed. These fluctuations appear in thermally-assisted recording as increases in the recording jitter, decreases in the signal-to-noise ratio (SNR), changes in the recorded track width, and increases in the interference between adjacent tracks—adjacent track interference (ATI). Due to these undesirable effects, the error rate increases.
As a result, the recording density that is achievable using conventional thermally-assisted recording heads decreases substantially. In other words, power fluctuations caused by mode hopping became a large barrier to the implementation of high-density recording by thermal assistance.